Why high nighttime temperatures are bad news for Delhi

Delhi is grappling with a deadly heatwave. The National Capital has been witnessing temperatures above 40 degrees Celsius for days now. But what has surprised many are the record night temperatures.

On Tuesday (18 June), Delhi reported its warmest night in at least 12 years, with the mercury touching 35.2 degrees Celsius, according to the Indian Meteorological Department (IMD). This is the city’s highest minimum temperature since 1969. As per the weather department’s data, Tuesday’s night temperature surpassed the previous high of 34.9 degree Celsius recorded on 23 May 1972.

Until Thursday (20 June), Delhi saw maximum temperatures over 40 degrees Celsius for 37 consecutive days. After a spell of light rain on Wednesday night, the night temperature dropped to 29.6 degrees Celsius at the Safdarjung station, bringing some respite.

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The Delhi heatwave has been brutal on its residents, especially on outdoor workers. At least nine people have died of heatstroke in the National Capital in the last three weeks. Experts are saying high night temperatures have further made things worse.

Let’s take a look.

What are ‘warm nights’?

According to the IMD, a warm night is declared when the maximum temperature is above 40 degree Celsius and the minimum temperature is above normal by 4.5 to 6.4 degree Celsius.

A severe warm night is marked by a rise in minimum temperature from normal by more than 6.4 degree Celsius.

While Delhi clocked a maximum temperature of 42 degrees Celsius on Tuesday, the heat index – the feeling of heat on the human body – was 51 degree Celsius, as per The National.

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Delhi’s Tuesday night temperature was eight degrees above normal.

The National Capital’s power demand reached a record high of 8,647 megawatts (MW) on Tuesday afternoon. This further increased to 8,656 MW on Wednesday afternoon.

Besides Delhi, several regions over northwest India and east India including Punjab, Haryana, West Uttar Pradesh and Bihar have reported warm to severe warm night conditions in May and June.

Why warm nights are a big concern

High temperatures at night indicate the Earth which is getting boiled in the day is failing to dissipate heat even hours after sunset.

After bearing sweltering temperatures throughout the day, the human body needs the night to recuperate. But due to warm nights, even that is not happening, leading to higher incidents of heat-related illnesses.

“The reason that more heat stroke cases are being reported now, despite the maximum temperature hitting its peak in the last week of May, is the rise in night temperatures. There is no respite at all” a doctor at a Delhi government hospital told Indian Express on the condition of anonymity.

“Moreover, homes are also warmer than the outdoors at night. So people are out in the open when it is the hottest outside [during the day] and indoors when the dip in temperature is not very much,” the doctor added.

According to a recent analysis by Gujarat-based Indian Institute of Public Health (IIPH), people who do not have access to cooling are more vulnerable to health issues due to high night temperatures.

“This happens because the body doesn’t get rest. Normally, night is the time to recuperate from the impact of extreme heat, but if you do not get rest at night, then your body will be in overdrive. For example, if I ask someone to run and instead of giving rest intermittently, I say you can only slow to a brisk walk and then run again, the person will collapse. The same thing is happening when day and night temperatures are both extremely high,” Dr Dileep Mavalankar, former director at IIPH Gandhinagar, told Hindustan Times (HT).

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What’s behind warm nights in Delhi?

Climate change has made heatwaves longer and more intense. Experts also attribute blistering heat to the El Niño phenomenon which is the unusual warming of the ocean surface in the central and eastern Pacific Ocean.

Experts flag the loss of green cover and water bodies and mindless concretisation in urban regions of Delhi-NCR for its summer woes, reported PTI.

Rajneesh Sareen, who leads the Sustainable Habitat Programme at Delhi-based think tank Centre for Science and Environment, told NDTV, “The construction and concretisation in big cities like Delhi have gone up significantly. Concrete buildings absorb heat through the day and release it at night. This is why minimum temperatures are rising in big cities.”

He said “ACs are exploding” because of the heat island effect.

The ‘urban heat island effect’ is usually seen in metropolitan areas that are significantly hotter than their surroundings which are relatively more open and greener.

Speaking to HT, Vishwas Chitale, senior programme lead, Council on Energy, Environment and Water (CEEW) raised alarm that climate crisis would trigger more severe heatwaves.

“These high nighttime temperatures pose serious health concerns, as they prevent the earth from cooling down after the daytime heat. It is becoming increasingly clear that due to climate change, boiling summers are the new normal and that we need year-round preparedness to be more proactive than reactive in dealing with heatwaves,” Chitale said.

With inputs from agencies